This invention is related to etalon-type optical filters and, in particular, to tunable optical filters in which optical fiber ends are used to define the etalon cavity.
Optical filters which are tunable are highly desirable components for optical networks and for some optical systems, such as spectrometers, for example. Optical filters typically transmit light at particular wavelengths and block light at other wavelengths. An optical filter which is tunable allows the wavelengths to be selected.
Tunable optical filters can be quite varied in construction. For example, liquid crystal cells, thin film filters, ruled gratings, distributed Bragg reflectors (gratings), and fiber Bragg gratings, a type of distributed Bragg reflector, have been used as the tuning constituents in tunable optical filters. Of course, other parts are required to operate the tuning constituent in a tunable optical filter. The resulting optical performance, reliability, speed of operation, cost and size, among many parameters, of such tunable optical filters vary widely.
One conventional (and simple) structure for an optical filter is the etalon, also called a Fabry-Perot interferometer, in which two highly reflective, parallel surfaces form a resonating cavity for wavelength selection. To make the etalon tunable, the optical distance between the two reflecting mirrors is changed. One type of etalon, or Fabry-Perot interferometer, tunable optical filter uses the end surfaces of optical fibers as the reflecting surfaces of the etalon. However, the large numerical aperture (NA) of the optical fibers and resulting beam divergence cause a large insertion loss to the detriment of optical performance. One way of minimizing such losses is to use a concave surface at the end surface of the one of the optical fibers. Nonetheless, the insertion loss can still be lowered. These fiber-ended tunable optical filters also have significant sideband or side mode peaks in the transmission spectra which adversely affect the performance of the filters. Furthermore, current fiber etalon-type tunable optical filters remain expensive and their applications to real world problems are accordingly reduced.
The present invention is directed toward avoiding these problems, improving the optical performance of fiber etalon-type, tunable optical filters, and lowering their costs to expand their applications.